What you can do is make a wrapper for the index.php file of your wordpress installation. The wrapper handles every single visit and shows it a “Under Construction” or “Under maintenance” page but you can browse the site as if it where live. All with out redirects, htaccess hacks or copying files from on folder to another. You can get a sample file and the code at http://www.fuseboxcreations.com/2010/tutorials/how-to-set-up-a-wordpress-under-construction-environment/. Let us know if it helps!
